Article Geochemistry 3 Volume 9, Number 1 Geophysics 26 January 2008 Q01003, doi:10.1029/2007GC001677 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Effect of three-dimensional slab geometry on deformation in the mantle wedge: Implications for shear wave anisotropy Erik A. Kneller and Peter E. van Keken Department of Geological Sciences, University of Michigan, 2534 CC Little Building, 1100 North University Avenue, Ann Arbor, Michigan 48109, USA ([email protected]; [email protected]) [1] Shear-wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench-parallel fast directions. Three-dimensional flow may give rise to trench-parallel stretching and provide an explanation for these patterns of seismic anisotropy. Along-strike variations in slab geometry produce trench-parallel pressure gradients and are therefore a possible mechanism for three-dimensional flow. In this study we quantify the effects of variable slab dip, curved slabs, oblique subduction, and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones. Temperature, dynamic pressure, velocity, and strain are calculated with high-resolution three-dimensional finite element models. These models include temperature- and stress-dependent rheology and parameterized slab and trench geometry. Thick layers (20–60 km) with strong trench-parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15° or strong curvature in the slab. In these cases, strong trench-parallel stretching develops when flow lines have an oblique to trench-normal orientation. This suggests that trench-parallel seismically fast directions may not indicate trench-parallel flow lines in systems with large along-strike variations. An oblique component of stretching is confined to a 20–30 km layer above the slab in systems with oblique subduction. The effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench-parallel flow that extends 50–100 km into the core of the mantle wedge. Components: 8135 words, 16 figures, 3 tables. Keywords: three-dimensional subduction zone dynamics; shear wave splitting; non-Newtonian rheology. Index Terms: 8162 Tectonophysics: Rheology: mantle (8033); 8413 Volcanology: Subduction zone processes (1031, 3060, 3613, 8170); 8120 Tectonophysics: Dynamics of lithosphere and mantle: general (1213). Received 5 May 2007; Revised 12 August 2007; Accepted 18 October 2007; Published 26 January 2008. Kneller, E. A., and P. E. van Keken (2008), Effect of three-dimensional slab geometry on deformation in the mantle wedge: Implications for shear wave anisotropy, Geochem. Geophys. Geosyst., 9, Q01003, doi:10.1029/2007GC001677. 1. Introduction et al., 1998] and likely affects thermal structure, which is a fundamental control on arc magma [2] Three-dimensional solid-state creep may play genesis [Ulmer, 2001] and slab metamorphism an important role in a variety of processes that [Hacker et al., 2003]. Three-dimensional deforma- occur in the mantle wedge of subduction zones. tion may also control rock fabric development and Three-dimensional flow may control along-strike patterns of seismic anisotropy [Hall et al., 2000; geochemical trends observed in some arcs [Ewart Park and Levin, 2002; Mehl et al., 2003]. The common observation of trench-parallel seismically Copyright 2008 by the American Geophysical Union 1 of 21 Geochemistry 3 kneller and van keken: effect of three-dimensional slab geometry Geophysics 10.1029/2007GC001677 Geosystems G Figure 1. Seismic anisotropy around the Pacific Ring of Fire. Black arrows denote the fast directions of split local-S phases above subducting slabs (see auxiliary material Figures S1–S4). Blue arrows denote teleseismic shear wave splitting [Wolfe and Solomon, 1998]. Red lines show the fast direction measured from surface-wave and Pn tomography [Park and Levin, 2002]. Note that fast directions at spreading centers are consistently ridge normal (plate motion parallel), whereas at subduction zones fast directions are mostly trench-parallel or perpendicular to plate motion. Symbols denote regions with large along-strike variations in slab geometry and type of geometry: C denotes curved slabs, D represents variable dip, E denotes slab edges, and O refers to oblique subduction similar to case 9. Superscripts denote cases presented in this study with slab geometry that is applicable to a particular subduction system. fast directions in subduction zones (Figure 1) has vations at most subduction zones show complex been used to infer the presence of a significant patterns of seismic anisotropy that are not consis- amount of trench-parallel stretching associated tent with this model [Wiens and Smith, 2003] with trench-parallel flow in the mantle wedge. In (Figure 1 and auxiliary material Figures S1–S4).1 this study we investigate the role of along-strike These complex patterns commonly include abrupt variations in slab geometry in producing three- rotations in seismically fast directions [Polet et al., dimensional flow and trench-parallel stretching in 2000; Nakajima and Hasegawa, 2004; Anderson et the mantle wedge. al., 2004; Long and van der Hilst, 2005; Nakajima et al., 2006] and trench-parallel fast splitting with [3] Two-dimensional cornerflow models of defor- maximum delay times around 1 s [Smith et al., mation in the mantle wedge predict strong stretch- 2001; Anderson et al., 2004; Long and van der ing parallel to plate motion with maximum trench- Hilst, 2005, 2006]. Auxiliary material Figures S1– normal stretching exceeding 500% [McKenzie, S4 show shear-wave splitting patterns and three- 1979; Fischer et al., 2000] (Figure 2). Commonly dimensional slab geometry in Pacific subduction observed varieties of olivine fabric show flow- systems. parallel anisotropy with seismically fast directions that align parallel to the maximum stretch direction [4] Several hypotheses have been proposed that or approximately parallel to the flow direction for provide an explanation for the common observa- simple two-dimensional flow. The pattern of de- tion of trench-parallel seismically fast directions in formation shown in Figure 2 with flow-parallel olivine fabric will give rise to seismically fast directions that align parallel to the trench [Ismaı¨l 1Auxiliary materials are available in the HTML. doi:10.1029/ and Mainprice, 1998]. Shear-wave splitting obser- 2007GC001677. 2of21 Geochemistry 3 kneller and van keken: effect of three-dimensional slab geometry Geophysics 10.1029/2007GC001677 Geosystems G and back-arc mantle of subduction zones. Candi- date three-dimensional flow mechanisms include small-scale convection [Honda et al., 2002; Honda and Saito, 2003; Honda and Yoshida, 2005; Behn et al., 2007], differential slab rollback [Russo and Silver, 1994; Mehl et al., 2003; Anderson et al., 2004], oblique subduction [Hall et al., 2000; Mehl et al., 2003; Honda and Yoshida, 2005], trench- parallel motion of the overriding plate [Hall et al., 2000], slab-edge effects [Kincaid and Griffiths, 2003; Piromallo et al., 2006], large-scale mantle flow [Smith et al., 2001; Lowman et al., 2007], and variable slab geometry [Hall et al., 2000]. Geo- dynamic modeling can be used to investigate the Figure 2. The evolution of finite strain ellipses along sensitivity of these mechanism to subduction several streamlines for analytical cornerflow. Slab- parameters and applicability to specific subduction driven flow produces a low-pressure corner and systems. However, a limited amount of work has channelized return flow (gray region). The inflow been done in full three-dimensional geometry. channel is defined as a channel-like region where material flows from the back arc mantle to the arc [6] Analogue experiments show that slab rollback mantle where dynamic pressure is relatively low. Note is associated with limited trench-parallel stretching that flow lines in the core of the inflow channel are when slabs converge into the mantle [Buttles and associated with relatively small amounts of shear and Olson, 1998; Kincaid and Griffiths, 2003]. How- stretching. Strain geometry within this region will be ever, these experiments suggest that significant most affected by trench-parallel pressure gradients. toroidal flow and trench-parallel stretching may occur close to slab edges. Recent numerical models of large-scale flow in subduction zones also show the mantle wedge. These hypotheses use one of the strong toroidal flow next to slab edges in addition following mechanisms: (1) olivine fabric transi- to complicated flow in the mantle wedge [Lowman tions [Katayama and Karato, 2006; Kneller et al., et al., 2007]. Three-dimensional models of free 2005, 2007; Lassak et al., 2006], (2) preferred convection in the mantle wedge suggest that water orientation of melt-filled cracks or melt networks weakening [Honda et al., 2002; Honda and Saito, [Hiramatsu et al.,1998;Fischer et al.,2000; 2003; Honda and Yoshida, 2005] and crustal Holtzman et al., 2003], and (3) three-dimensional foundering [Behn et al., 2007] may give rise to flow with commonly observed olivine fabric [Hall three-dimensional flow beneath volcanic arcs. et al., 2000; Mehl et al., 2003; Behn et al., 2007; These models have yet to include three-dimension- Lowman et al.,2007].AtransitiontoB-type al finite strain calculations and fabric development, olivine fabric with flow-normal